This invention relates to diode lasers, and more particularly to the construction of wide strip diode lasers.
A diode laser is commonly created with a GaAs substrate which supports an electrical conductor and which has two parallel faces that are perpendicular to the substrate surface. An epitaxially grown layer of between 1 and 5 quantum wells is grown on the substrate surface and a strip that extends between the two faces is generally created by etching the quantum wells layer. A second epitaxial layer which also supports a second electrical conductor is grown (typically, also GaAs) to cover the quantom well strip. The two parallel faces are highly polished; with one being partially reflective and the other being either partially or highly reflective.
When an electrical current is forced through the conductors, the structure lases and light is emitted from the edge of the structure. Specifically, light is emitted from the partially reflective face of the structure in the area that corresponds to the cross section of the quantum wells strip. The light intensity is a function of the current level, but that light cannot be increased without bound by merely raising the current level because when the light is too intense it destroys the epitaxial material and/or the substrate. The overall light power that the diode laser emits can be increased, however, by widening the quantum wells strip.
The light emitted from a diode laser with a structure as described above does not have a circular cross section. Rather, it corresponds more closely to the shape of the cross section of the quantum wells strip. Consequently, diode lasers that achieve a high optical output through a wider strip possess a light beam that is far from circular. Moreover, the light from such a structure diverges very quickly in the plane perpendicular to the plane of the strip but diverges slowly in the plane of the strip. Thus, the light starts out roughly with a rectangular cross section having the long edge corresponding to the long edge of the strip, and as it travels away from the diode laser it eventually takes on an approximately rectangular cross section with a long edge that is perpendicular to the long edge of the strip.
At the CLEO '91 conference in Baltimore, Md., May 12-27, 1991, Snyder et al. reported on an approach to arrest the quick divergence of the beam in the perpendicular plane of the diode laser structure described above. "Cylindrical Microlenses for Collimating Laser Diodes", Snyder et al., Conference Proceedings, pp. 28-31. Their proposed structure comprises an optical fiber segment that is attached to the edge of the diode structure in parallel to the strip, as shown in FIG. 1. Diode laser 5 includes a quantum wells strip 10 spanning a substrate from partially reflective facet 20 to highly reflective facet 30. In FIG. 1, only the cross section of strip 10 at surface 20 is shown. A cylindrical lens 40 that is made of optical fiber is glued to the substrate at the light emitting output area of strip 10 (i.e., at facet 20) and that lens substantially collimates the diverging light in the vertical plane corresponding to the y-axis direction in FIG. 1. It does not collimate the light perpendicular to the diverging light corresponding to the z-axis direction in FIG. 1.
For applications where high intensities and small spot sizes are desired, there are three problems associated with this approach. First, the glue used to attach the lens to the substrate lowers the light intensity at which physical damage to the diode laser begins to show up. That occurs primarily because the glue absorbs light energy that is converted to heat. Second, the lens reduces the divergence rate of the rapidly expanding beam but it does not create a circular beam which may be focused to a small spot size. Third, the wider strip permits multimode operation of the laser and that results in variation in the light intensity along the strip. The latter two problems increase the minimum spot size that can be achieved from the diode laser and, together, the three problems cause the maximum focused intensity to be decreased.